The present invention discloses a recombinant modified extracellular chitinase having an amino acid sequence set forth in SEQ ID NO. 4 or SEQ ID NO. 5 prepared by substituting two tyrosine (Y) residues with histidine (H) in the native chitinase of Brevibacillus laterosporus LAK 1210. The modified chitinase represents an advancement as it has improved thermal stability, wider optimum pH range, high efficacy, improved solubility and low toxicity. The present invention also provides compositions and improved methods for producing and purifying the recombinant modified chitinase by chitin affinity chromatography and chitin adsorption affinity chromatography using shrimp shell and crab shell, for large scale production at low cost. The modified chitinase has wide range of applications including prevention, treatment or modulation of phytopathogenic infection in a plant or a plant part.